This invention relates to a method of preparing alumina which is useful for use as a catalyst carrier.
Alumina is now used in a wide variety of fields. .gamma.-Alumina is suitably used as a catalyst carrier because of its large surface area, excellent mechanical strength and ability of supporting catalytic metals uniformly on its surface. .gamma.-Alumina may be generally obtained by calcining pseudo-boehmite, which is a hydrogel of fine, fibrous boehmite crystallites, at a temperature of about 600.degree. C. The calcination of .gamma.-alumina at a temperature of 900.degree.-1000.degree. C. gives .delta.-alumina. At a calcination temperature of 1100.degree. C., .gamma.-alumina is converted into .alpha.-alumina.
Many methods have thus far been proposed for the preparation of alumina. One such method includes aging an aqueous slurry containing seed aluminum hydroxide at a pH of 6-11 for the growth of the seed crystals by coalescence. This method requires a long period of time to obtain hydrogel particles of a large size.
U.S. Pat. Nos. 4,248,852 and 4,422,960 disclose a method for the preparation of alumina suitably used as catalyst carrier, wherein first and second pH controlling agents are alternately and repeatedly mixed with an aqueous slurry containing seed aluminum hydroxide to swing the pH of the slurry between hydrogel dissolution and precipitation regions. At least one of the first and second pH controlling agents includes an aluminum compound capable of forming an alumina hydrogel. Since aluminum hydroxide is continually replenished during the hydrogel growing step, the rate at which the seed aluminum hydroxide grows in size is much higher than that in the method in which the growth of the seed particles is effected by mere coalescence of the seed particles. However, this method has been found to involve a problem in that the resulting alumina carrier does not have entirely satisfactory chemical and physical stability.